Re: [PATCH 05/16] KVM: x86/mmu: Use synthetic page fault error code to indicate private faults

From: Sean Christopherson <seanjc@google.com>
To: Kai Huang <kai.huang@intel.com>
Cc: "pbonzini@redhat.com" <pbonzini@redhat.com>,
	 "yu.c.zhang@linux.intel.com" <yu.c.zhang@linux.intel.com>,
	 "chao.p.peng@linux.intel.com" <chao.p.peng@linux.intel.com>,
	 "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"tabba@google.com" <tabba@google.com>,
	 Yan Y Zhao <yan.y.zhao@intel.com>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	 "michael.roth@amd.com" <michael.roth@amd.com>,
	Isaku Yamahata <isaku.yamahata@intel.com>,
	 "dmatlack@google.com" <dmatlack@google.com>
Subject: Re: [PATCH 05/16] KVM: x86/mmu: Use synthetic page fault error code to indicate private faults
Date: Thu, 29 Feb 2024 07:17:14 -0800	[thread overview]
Message-ID: <ZeCf-sJIyR7vuMtD@google.com> (raw)
In-Reply-To: <063aa825af395439cc1b3669fb326c395bd6fe42.camel@intel.com>

On Thu, Feb 29, 2024, Kai Huang wrote:
> 
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 408969ac1291..7807bdcd87e8 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -5839,19 +5839,31 @@ int noinline kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, u64 err
> >  	bool direct = vcpu->arch.mmu->root_role.direct;
> >  
> >  	/*
> > -	 * IMPLICIT_ACCESS is a KVM-defined flag used to correctly perform SMAP
> > -	 * checks when emulating instructions that triggers implicit access.
> >  	 * WARN if hardware generates a fault with an error code that collides
> > -	 * with the KVM-defined value.  Clear the flag and continue on, i.e.
> > -	 * don't terminate the VM, as KVM can't possibly be relying on a flag
> > -	 * that KVM doesn't know about.
> > +	 * with KVM-defined sythentic flags.  Clear the flags and continue on,
> > +	 * i.e. don't terminate the VM, as KVM can't possibly be relying on a
> > +	 * flag that KVM doesn't know about.
> >  	 */
> > -	if (WARN_ON_ONCE(error_code & PFERR_IMPLICIT_ACCESS))
> > -		error_code &= ~PFERR_IMPLICIT_ACCESS;
> > +	if (WARN_ON_ONCE(error_code & PFERR_SYNTHETIC_MASK))
> > +		error_code &= ~PFERR_SYNTHETIC_MASK;
> >  
> 
> Hmm.. I thought for TDX the caller -- handle_ept_violation() -- should
> explicitly set the PFERR_PRIVATE_ACCESS so that here the fault handler can
> figure out the fault is private.
> 
> Now it seems the caller should never pass PFERR_PRIVATE_ACCESS, then ...
> 
> >  	if (WARN_ON_ONCE(!VALID_PAGE(vcpu->arch.mmu->root.hpa)))
> >  		return RET_PF_RETRY;
> >  
> > +	/*
> > +	 * Except for reserved faults (emulated MMIO is shared-only), set the
> > +	 * private flag for software-protected VMs based on the gfn's current
> > +	 * attributes, which are the source of truth for such VMs.  Note, this
> > +	 * wrong for nested MMUs as the GPA is an L2 GPA, but KVM doesn't
> > +	 * currently supported nested virtualization (among many other things)
> > +	 * for software-protected VMs.
> > +	 */
> > +	if (IS_ENABLED(CONFIG_KVM_SW_PROTECTED_VM) &&
> > +	    !(error_code & PFERR_RSVD_MASK) &&
> > +	    vcpu->kvm->arch.vm_type == KVM_X86_SW_PROTECTED_VM &&
> > +	    kvm_mem_is_private(vcpu->kvm, gpa_to_gfn(cr2_or_gpa)))
> > +		error_code |= PFERR_PRIVATE_ACCESS;
> > +
> > 
> 
> ... I am wondering how we figure out whether a fault is private for TDX?

Read the next few patches :-)

The sanity check gets moved to the legacy #PF handler (any error code with bits
63:32!=0 yells) and SVM's #NPF handler (error code with synthetic bits set yells),
leaving VMX free and clear to stuff PFERR_PRIVATE_ACCESS as appropriate.